RFR (S) 8213092: Add more runtime locks for concurrent class unloading (original) (raw)

David Holmes david.holmes at oracle.com
Tue Oct 30 12:36:54 UTC 2018

On 30/10/2018 10:24 PM, coleen.phillimore at oracle.com wrote:

On 10/30/18 12:17 AM, David Holmes wrote:

Hi Coleen,

On 30/10/2018 1:45 PM, coleen.phillimore at oracle.com wrote: Summary: Add locks for calling CLDG::purge concurrently as well and for calling SystemDictionary::dounloading concurrently.

Ran linux-x64 tier1-6 through mach5 and hotspot/jtreg/runtime tests, which include the module tests. open webrev at http://cr.openjdk.java.net/~coleenp/8213092.01/webrev bug link https://bugs.openjdk.java.net/browse/JDK-8213092 So ... all the locks covered by an assertlockedorsafepoint, or which are acquired by the VMThread at a safepoint, must never be held by a JavaThread if it could reach a safepoint whilst that lock is held - else we could deadlock. So can we check that with NoSafepointVerifiers? Actually I think this is not possible to add NSV.   You can acquire the ClassLoaderDataGraphlock and then the Modulelock.  The latter would check for a safepoint also for a Java thread.  This is currently done for Jvmti and JFR, but not in other code that I can see.  I don't actually know how to fix this problem.

This seems risky. If a JavaThread can hold the CLDG_lock while blocked at a safepoint (acquiring the Module_lock), then what is to stop the VMThread from hitting one of these sections of code protected by locked_or_safepoint and then proceeding into what is effectively a critical section (by virtue of there being a safepoint) when the JavaThread is itself in the midst of a critical section? Do we actively take steps to prevent this somehow, or to make it safe for the VMThread to proceed?

The locks added in this patch set though are for the NonJavaThreads, who do not do safepoint checks.  The NonJavaThreads that acquire these locks use the STS joiner mechanism which disallows safepoints while being held (and since they are non Java threads, do not check for safepoints themselves). This is how it's going to look for the ZGC caller: { SuspendibleThreadSetJoiner stsjoiner; // Unlink the classes. MutexLockerEx ml(ClassLoaderDataGraphlock); unloadingoccurred = SystemDictionary::dounloading(ZStatPhase::timer(), true /* docleaning */);); }

Somehow I missed the creation/invention of the STS joiner mechanism.

David

Further, are these locks acquired by non-JavaThreads such that the VMThread may be delayed whilst a safepoint is active? Yes, theoretically they could delay the VMThread from getting to a safepoint or doing its work while in a safepoint but the threads that take out these locks only hold them for short durations. Thanks, Coleen Thanks, David Thanks, Coleen

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